This invention relates to a process for the manufacture of through-hole contacted flexible circuit boards which are suited for high bending stresses and the circuit boards derived therefrom.
Flexible circuit elements are commonly used in electronic components that are subjected to high dynamic bending stresses, such as, for example, in disk storages and printers. These flexible circuit elements must have a smooth and extremely clean surface, in order to avoid disturbances of the electromechanical functions (of the disc drive or other electronic component) as the result of particles released from the flexible circuit board; particularly particles released from the adhesive materials used in manufacturing the flexible circuitry.
Circuit boards for high bending stresses which have two circuit layers with through-hole contacts located outside the bending area, are usually designed to be flexible on one side and to have carrier and covering films of equal thickness (symmetrical laminar construction). The base material used is a carrier film, made, for example, of polyimide, with copper-laminated on both sides thereof. Generally, the copper layer or foil is attached to the carrier film by means of an adhesive layer. The formation of through-hole contacts and a conductive pattern (on the copper foil) is accomplished in a conventional manner by a subtraction process (etching). As a result, in the bending area, the conducting paths (circuit pattern) are formed on only one side thereof, and the other side is completely etched away. The conductors are then provided with a protective covering film, of the same thickness as the carrier film.
Despite the apparent symmetrical construction of the above-described flexible circuits, the conducting paths thereon are not located in the transverse section of the laminate that remains "neutral" when subjected to bending stress, that is, the section that is neither subjected to tensile or compressive strains (the "neutral section"). In fact, the adhesive layer of the carrier film (which has been etched away) will produce an asymmetry, which acts to displace the neutral section into other transverse areas. In addition, as a result of the exposed adhesive, the circuit board is liable to accumulate dust and to be subjected to abrasion and thereby contaminate the electronic component.
Attempts have also been made in the manufacture of the above-described flexible circuits to make the carrier film of the base material only half as thick as the covering film on the printed circuit side.
Also, a covering film with the same thickness as the carrier film has been applied on the side of the carrier film that is etched away. While this particular construction has no exposed adhesive, unfortunately, it is still impossible to achieve a strict symmetry in the bending area, which is attributable to the existence of the two layers of adhesive on the carrier film. Furthermore, the total thickness of the construction in the bending area cannot be made thin enough, and consequently flexible enough, as is possible in the case of a one-sided circuit board.